The input system is a free form keyword lookup.

The following keywords are supported 

- 'FULL_OVERLAP' -- Specifies if overlap extents is all CFD cells []
- 'OVERLAP_EXTENTS' -- If FULL_OVERLAP is false, specify how much overlap
- 'CONSTRAINT_INFO' -- specify which part of the overlap is for sending constraint (CFD -> MD)
- 'BOUNDARY_EXTENTS' -- specify which part of overlap is for sending boundary (MD -> CFD)
- 'DENSITY_CFD' -- Density for use in coupling, often not specified explicitly by non-dimensional CFD
- 'TIMESTEP_RATIO' -- ratio of timesteps in both MD/CFD codes
- 'MATCH_CELLSIZE' -- Force cellsize to match in CFD and MD code
- 'CPL_CFD_BC_XYZ' -- Specifies which boundary condition to switch on or off in CFD
- 'CPL_CFD_BC_SLICE' -- Specifies if single value or cell by cell values taken in CFD
- 'CPL_MD_BC_SLICE' -- Specifies if single value or cell by cell values taken in MD
- 'SENDTYPE_MD_TO_CFD' -- Specify values to send from MD to CFD
- 'SENDTYPE_CFD_TO_MD' --Specify values to send from CFD to MD


These variables set internal values which can be retrieved with CPL_get( ... ). Which values are useful depend on the implementation of the coupled APPS and will often not need to be set by the user.

An example input file is included below 

###############################################################################
#                                                                             #
#           8888888 888b    888 8888888b.  888     888 88888888888            #
#             888   8888b   888 888   Y88b 888     888     888                #
#             888   88888b  888 888    888 888     888     888                #
#             888   888Y88b 888 888   d88P 888     888     888                #
#             888   888 Y88b888 8888888P"  888     888     888                #
#             888   888  Y88888 888        888     888     888                #
#             888   888   Y8888 888        Y88b. .d88P     888                #
#           8888888 888    Y888 888         "Y88888P"      888                #
#                                                                             #
###############################################################################

###############################################################################
#
#   density_cfd (float)
#     - specifies the density for both continuum fluid
#
# -----------------------------------------------------------------------------
DENSITY_CFD
0.5


###############################################################################
#
#   overlap_extents (6 * integers)
#     - define the global cell boundaries of the overlap region
#
#     1. icmin_olap
#     2. icmax_olap
#     3. jcmin_olap
#     4. jcmax_olap
#     5. kcmin_olap
#     6. kcmax_olap
#
# -----------------------------------------------------------------------------
OVERLAP_EXTENTS
1
64
1
4
1
64

##############################################################################
##
##   Specify constraint information
##
##      1. constraint_algo 0-off, 1-OCT, 2-NCER, 3-Flekkoy, etc
##      2. constraint_CVflag -- Use CV form of constraint 0-Off 1-On
##      3. icmin_cnst
##      4. icmax_cnst
##      5. jcmin_cnst
##      6. jcmax_cnst
##      7. kcmin_cnst
##      8. kcmax_cnst
##
## -----------------------------------------------------------------------------
CONSTRAINT_INFO
3
0
1
6
1
2
1
1

###############################################################################
#
#   timestep ratio (integer) 
#     - number of molecular time steps per continuum time step 
#
# -----------------------------------------------------------------------------
TIMESTEP_RATIO
50


###############################################################################
#
#   match cellsize (integer, flag)
#     - if on, force number of molecular cells (from the force-interaction 
#       cell-list optimisation) to be an integer multiple of the number of 
#       continuum cells 
#
#          0 = "off"
#          1 = "on"
#
# -----------------------------------------------------------------------------
MATCH_CELLSIZE
1

###############################################################################
#
#   Specify which components of velocity in CFD boundary condition are
#   obtained from the MD (3*integer: x,y,z)
#
#          0 = "off"
#          1 = "on"
#
# -----------------------------------------------------------------------------
CPL_CFD_BC_XYZ
1
1
1

###############################################################################
#
#   Logical flag for averaging MD velocity in spanwise direction for the
#   CFD boundary condition in the cfd socket. 
#
#          0 = cell-by-cell coupling
#          1 = cell velocities averaged in a spanwise plane
#
# -----------------------------------------------------------------------------
CPL_CFD_BC_SLICE
1

###############################################################################
#
#   Logical flag for averaging MD quantities in the constraint region
#
#          0 = averaging on a cell-by-cell basis
#          1 = extra averaging across a slice in the spanwise direction 
#
# -----------------------------------------------------------------------------
CPL_MD_BC_SLICE
0


New options can be added to CPL_module as follows, to create "NEWKEY" add the following to read_coupler_input(): 

call locate(infileid, 'NEWKEY', found)
if (found) then
    read(infileid, *) newkeyvalue
else
    newkeyvalue = 0
endif

where the type of newkeyvalue must be defined in the module at the top. 

    integer :: newkeyvalue

This can then be used internally or added to CPL_get as follows, 

subroutine CPL_get(icmax_olap,icmin_olap,jcmax_olap,jcmin_olap,  & 
                             ...   &
                   newkeyvalue)
    use coupler_module, only :  icmax_olap_=>icmax_olap,         &
                             ...  &
                   newkeyvalue_ => newkeyvalue

                             ...
    integer, optional, intent(out) :: newkeyvalue
                             ...

    if (present(newkeyvalue)) newkeyvalue = newkeyvalue_

end subroutine CPL_get

The wrappers for C, C++ and Python would also need to be added depending on which language the input needs to be available for.

This is a fairly cumbersome as changes like this are not expected to be common for most users. However, a JSON input system which allows arbitrary inputs is also provided, this can be build using JSON_SUPPORT flag but requires cmake.